Co-expression of soybean glycinins A1aB1b and A3B4 enhances their accumulation levels in transgenic rice seed

The soybean major storage protein glycinin is encoded by five genes, which are divided into two subfamilies. Expression of A3B4 glycinin in transgenic rice seed reached about 1.5% of total seed protein, even if expressed under the control of strong endosperm-specific promoters. In contrast, expression of A1aB1b glycinin reached about 4% of total seed protein. Co-expression of the two proteins doubled accumulation levels of both A1aB1b and A3B4 glycinins. This increase can be largely accounted for by their aggregation with rice glutelins, self-assembly and inter-glycinin interactions, resulting in the enrichment of globulin and glutelin fractions and a concomitant reduction of the prolamin fraction. Immunoelectron microscopy indicated that the synthesized A1aB1b glycinin was predominantly deposited in protein body-II (PB-II) storage vacuoles, whereas A3B4 glycinin is targeted to both PB-II and endoplasmic reticulum (ER)-derived protein body-I (PB-I) storage structures. Co-expression with A1aB1b facilitated targeting of A3B4 glycinin into PB-II by sequestration with A1aB1b, resulting in an increase in the accumulation of A3B4 glycinin.